In recent years there has been an increase in the number of products which require, as part of their fabrication process, the deposition of organic or inorganic soluble or dispersible materials such as polymers, dyes, colloid materials and the like on solid surfaces. One example of these products is an organic polymer electroluminescent display device. An organic polymer electroluminescent display device requires the deposition of soluble polymers into predefined patterns on a solid substrate in order to provide the light emitting pixels of the display device. Further examples include the deposition of materials for forming organic polymer thin film transistors (TFTs) on a substrate and interconnects between chips assembled on the substrate using fluidic self assembly (FSA). The substrate may, for example, be formed of glass, plastics or silicon.
Typically, the substrate is a rigid substrate, thereby providing a rigid display device. However, products comprising flexible displays, which may be rolled or folded, are increasingly sought after, in particular where a large display is required. Such flexible displays provide substantially improved weight and handling characteristics and are less likely to fail due to shock during installation of the display device or use of the display device. In addition, relatively small display devices comprising a large display area may be conveniently provided.
In the manufacture of semiconductor display devices, including light emitting diode (LED) displays, it has been conventional to use photolithographic techniques. However, photolithographic techniques are relatively complex, time consuming and costly to implement. In addition, photolithographic techniques are not readily suitable for use in the fabrication of display devices incorporating soluble organic polymer materials. Concerns relating to the fabrication of the organic polymer pixels have, to some extent, hindered the development of products such as electroluminescent display devices incorporating such materials to act as the light emitting pixel elements.
In addition, the use of etch masks, such as photo masks for photolithography or metal shadow masks for patterning by evaporation deposition, is well known in conventional fabrication techniques. Hence, these processes will not be described in detail in the context of the present invention. However, such conventional fabrication techniques present severe process concerns for a number of devices including large scale display devices. Indeed, the etching and deposition of relatively long but extremely narrow lines has, for a long period of time, presented severe fabrication difficulties as it is very difficult to produce mechanically robust masks which will provide the required definition in the finished product. For example, a metal shadow mask for evaporation deposition for a large scale display device will inevitably exhibit some sagging or bowing in the central unsupported portion of the mask. This leads to an uneven distance between the mask and the substrate at the edge and the centre of the substrate respectively, thereby giving rise to uneven width and thickness of the deposited lines and adversely affecting the quality of the display.
Organic semiconducting polymers may be printed in high resolution patterns using inkjet technology and are therefore an attractive alternative to the more conventional semiconductor materials, such as silicon, for the production of light emitting diodes for flat display panels and field effect transistors.
Consequently, it has been proposed to use inkjet technology to deposit the soluble organic polymers in the fabrication of, for example, electroluminescent display devices and thin film transistors. Inkjet technology is, by definition, ideally suited to the deposition of such soluble or dispersible materials. It is a fast and inexpensive technique. In contrast to alternative techniques such as spin coating or vapour deposition, it instantly provides patterning without the need for an etch step in combination with a lithographic technique. Furthermore, the high specification processing techniques, such as vacuum and deposition processing, are not required, as is the case for the fabrication of inorganic semiconductors. The investment in capital equipment to fabricate devices can therefore also be reduced. Additionally, when compared to a spin coating technique there is less waste of the organic material as the material is deposited in very small quantities directly as the requisite predefined patterns.
However, the deposition of the soluble organic materials onto the solid surface using inkjet technology differs from the conventional use of the technology, to deposit ink on paper, and a number of difficulties are encountered. In particular, there is a primary requirement in a display device for uniformity of light output and uniformity of electrical characteristics. There are also spatial limitations imposed in device fabrication. As such, there is the non-trivial problem to provide very accurate deposition of the soluble polymers onto the substrate from the inkjet print head. This is particularly so for colour displays as respective polymers providing red, green and blue light emissions are required to be deposited at each pixel of the display.
Substrate sizes can be relatively large and are typically 40 cm×50 cm or larger. To assist the deposition of the soluble materials it has been proposed to provide the substrate with a layer which includes a pattern of wall structures defined in a de-wetting material so as to provide an array of wells or elongate trenches, bounded by the wall structures, for receiving the material to be deposited. Such a patterned substrate will be referred to hereinafter as a bank structure. When organic polymers in solution are deposited into the wells, the difference in the wettability of the organic polymer solutions and the bank structure material causes the solution to self align into the wells provided on the substrate surface.
However, it is still necessary to deposit the droplets of organic polymer material in substantial alignment with the wells in the bank structure. Even when such a bank structure is used, the deposited organic polymer solution adheres to some extent to the walls of the material defining the wells. This causes the central area of each deposited droplet to have, at best, a thin coating of deposited material, perhaps as low as 10% of the material in comparison to the material deposited at the walls of the bank structure. The deposited polymer material at the centre of the wells acts as the active light emissive material in the display device and if the polymer material is not deposited in accurate alignment with the wells, the amount and therefore the thickness of the active light emissive material can be further reduced. This thinning of the active light emissive material is of serious concern because the current passing through the material in use of the display is increased which reduces the life expectancy and the efficiency of the light emissive devices of the display. This thinning of the deposited polymer material will also vary from pixel to pixel if deposition alignment is not accurately controlled. This gives rise to a variation in the light emission performance of the organic polymer material from pixel to pixel because the LEDs constituted by the organic material are current driven devices and, as stated above, the current passing through the deposited polymer material will increase with any decrease in the thickness of the deposited material.
This performance variation from pixel to pixel gives rise to non-uniformity in the displayed image, which degrades the quality of the displayed image. This degradation of image quality is in addition to the reduction in operating efficiency and working life expectancy of the LEDs of the display. It can be seen therefore that accurate deposition of the polymer materials is essential to provide good image quality and a display device of acceptable efficiency and durability, irrespective of whether a bank structure is provided.
FIG. 1 shows a conventional inkjet deposition machine 100 which can be used for rigid or flexible substrates. The machine comprises a base 102 supporting a pair of upright columns 104. The columns 104 support a transverse beam 106 upon which is mounted a carrier 108 supporting an inkjet print head 110. The base 102 also supports a platen 112 upon which may be mounted a substrate 114, which is typically glass and has a maximum size of 40 cm×50 cm. The platen 112 is mounted from the base 102 via a computer controlled motorised support or translation stage 116 for effecting movement of the platen 112 both in a transverse and a longitudinal direction relative to the inkjet print head, as shown by the axes X and Y in FIG. 1. As the movement of the platen 112, and hence the substrate 114 relative to the inkjet head 110 is under computer control, arbitrary patterns may be printed onto the substrate by ejecting appropriate materials from the inkjet head 110 onto predetermined positions on the substrate. The computer control is further used to control the selection and driving of the nozzles and a camera may be used to view the substrate during printing. To enhance the accuracy of printing, position feedback may be provided for the translation stage, thereby allowing the position of the platen to be continually monitored during motion. In addition, a signal used for communicating between the translation stage and computer control can be used as a clock for timing inkjet ejection.
Two distinct techniques can be implemented to synchronise the position of the droplets to the substrate. One technique is the use of a signal as a trigger source for the timing of the ejection according to the velocity of the substrate. By matching the frequency of the ejection from the head to this velocity, a certain deposition spacing of droplets can be achieved. By changing the ratio of the two, the spacing between deposited droplets can be varied. Alternatively, another technique involves the use of the signal used in a position encoder system implemented in the translation stage. The position encoder is used in the translation stage to accurately determine the position of the moving platen. The position encoder sends a signal to the controller as a series of electrical pulses, the position and velocity of the stage is determined from this signal. This signal can therefore also be implemented as the timing signals for the inkjet head.
In either of the above cases, there is a requirement that the position of the head to the substrate is accurate to within microns to obtain uniform patterning of the material on to the substrate with the desired accuracy. To achieve this, accurate control of the positioning of the stage is crucial.
However, positional errors arising from mechanical limitations of the translation stage can occur, which can limit the positional accuracy of the inkjet print head 110 relative to the platen 112 and therefore the substrate 114 on which the high resolution patterning is required. Such limitation in positional accuracy may arise from the following exemplary causes.
The translation of the stage, and hence the platen, along its path may be erroneous, i.e. the distance actually translated by the stage may be marginally longer or shorter than the required distance which has been programmed into the machine. This can be explained with reference to FIG. 2, where the intended translation space is shown by the solid line rectangle defined by the points A, B, C, D, i.e. the actual points on the substrate which must be reached by the inkjet head; and the actual translation space, which is shown by the dotted line parallelogram defined by the points A, B′, C′, D′ and resulting from errors in the translation length and construction angle θ between the x and y axes of the translation system.
These errors in the translation length may occur in one or both of the axes shown in FIG. 2 and it can be seen from FIG. 2 that instead of the translation length from the point A (origin point) being either x or y, for example, the actual translation may be x+Δx or y+Δy. Errors may also be anticipated resulting from the combination of the two axes in an x-y configuration, where there may be an error in the construction angle subtended by the two axes. For printing of an accurate pattern, the angle subtended by the two axes should be exactly 90° but frequently this is not the case due to the manufacturing tolerances of the inkjet machine. If the subtended angle is not exactly 90°, it is to be expected that the position of the stage away from the origin point A will be in error and, at substantially large displacements from the point A, the erroneous positioning of the stage would be likely to cause unacceptable offsets in the deposited droplets from the inkjet head.
It will be appreciated that preparation alignment of the translation stage relative to the print head is required prior to actual deposition of droplets from the inkjet head to ensure that the translation stage and the head are aligned in both the x and y directions throughout the intended translation space, as defined by the points A, B, C and D in FIG. 2.